Update type hints

[mjpieters]

• explicit_port and __bytes__ were missing
• joinpath() takes an optional encoded argument

[codecov]

Codecov Report

Merging #891 (d61a106) into master (c20808a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #891   +/-   ##
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  Coverage   99.74%   99.74%           
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  Files           4        4           
  Lines         772      772           
  Branches      219      219           
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  Hits          770      770           
  Misses          2        2           

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unit	99.61% <ø> (ø)

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[mjpieters]

Preferably we detect these issues earlier, but with the current .pyi file I am not sure that that's possible. Moving the type hints into yarl/_url.py would make it possible to run mypy in strict mode and so catch missing type hints there, but that's a larger undertaking.